1. Description of the Field of the Invention
The principles for the generation of power by a nuclear reactor have been well established and are well understood. Briefly, the reactor contains uranium or plutonium fuel elements in a core arrangement. Through the mechanisms of neutron absorption and nuclear fission of the uranium or plutonium, large amounts of energy are released. This released energy manifests itself in the form of heat which is utilized to generate electricity. The heat is transferred to a primary coolant which continuously circulates through the core and carries the generated heat to a heat exchange boundary where a secondary coolant or working fluid is heated. Ordinarily the secondary coolant is water and is vaporized at the heat exchange boundary to produce steam. The steam is then circulated in a secondary system to a load for its ultimate use. In the power reactor context, the load is a turbine which is caused to turn at a predetermined rate. The turbine is then connected to a generator for the ultimate transformation of the thermal energy into electrical energy.
All elements of this system are functionally interrelated. As an example, an increase in reactor power increases the rate of energy transfer to the primary coolant which, in turn, increases the rate of energy transfer to the secondary coolant causing more energy provided to the load for its ultimate transformation into electrical energy. Conversely, if less electrical energy is required, the energy requirement of the turbine diminishes. The steam flow to the turbine is reduced and consequently, the turbine utilizes less of the thermal energy being transferred to the secondary coolant and an energy backup results. Since less energy is being drawn from the steam supply system when the steam flow is reduced, both the temperature and pressure of the steam generator secondary side are caused to increase. The effect of this increase in secondary coolant temperature is reflected on the primary side of the heat exchanger since less energy can be transferred across the heat exchange boundary. Accordingly, both the primary coolant's temperature and pressure increase. This trend continues until the reactor regulating system, which is programmed to keep the average temperature of the primary coolant on a specified program, returns the system to acceptable values by cutting back on the reactor's power by driving regulating rods into the core.
Since the regulating rods can only be slowly advanced into the core at a limited maximum speed, the reactor regulating system is unable to prevent a serious increase in primary and secondary pressures and temperatures if the magnitude and rate of energy backup described above exceed certain values. Among other things the energy backup is dependent on the magnitude and rate of decrease in load: called a load rejection. Ordinarily, the reactor and steam supply systems are designed to be able to withstand a load rejection of approximately ten percent and a rate of load rejection of five percent per minute (see FIG. 1), but if the load rejection or rate of load rejection exceed these values, the reactor regulating system is unable to compensate rapidly enough for the energy backup and the temperature and pressure of the primary system increase uncontrollably. When this occurs, protective systems come into operation to trip the reactor and/or to open steam dump and load bypass valves in order to avoid an overpressurization in the primary and secondary systems. If the uncontrolled increase in pressure is not avoided by these measures, the safety pressure valves of either the primary or the secondary side are caused to lift. This is an undesirable occurrence since it generally puts the system out of operation until the seals of the safety valves have been remachined and reseated. Another undesirable effect is that the reactor is tripped unnecessarily upon a large or rapid load rejection that otherwise would not require taking the reactor out of operation. Such a trip temporarily removes the nuclear power plant as a supplier and a time consuming and expensive reactor startup procedure must be followed before the reactor can be put back into operation as a power producer.
To better understand the dynamics of the secondary system, it is helpful to refer to curve 1 of FIG. 2 which showns the typical steady state functional relationship between the secondary pressure and reactor power or load. When the reactor power is at zero, the steam generator hot standby secondary pressure is maintained at a maximum value of 1,000 psi. When the reactor power is increased through the values from 0 to 100%, the secondary pressure falls from its initial value of 1,000 psi to a final value of 900 psi. The ordinary secondary coolant system protection system safety valves would have a fixed pressure setpoint of about 1100 psi. This setpoint is designed to protect the integrity of the secondary coolant system only and is not designed to protect the primary coolant system. The occurrence of a load rejection which involves either a large load decrease or a large rate of load decrease from a full power of 100% might raise the secondary pressure off of the steady state curve to a value which remains below the fixed secondary pressure setpoint while at the same time causing the pressure in the primary coolant system to increase to a unacceptable value which would require a reactor trip.
2. Description of the Prior Art
One previously employed method for protecting the coolant systems has been to monitor the primary pressure and either trip the reactor at a predetermined pressure setpoint or to lift the primary systems pressure safety valves. This method has the fundamental disadvantage that the reactor is forced to discontinue operation on the occurrence of a temporary load reduction with an interruption of the generation of electrical energy. One solution to this drawback is to over design the system to provide the capability of handling large pressure excursions. An additional disadvantage is that even when the reactor is tripped upon a high primary pressure, a pressure higher than the pressure setpoint cannot be avoided because of the thermal and mass inertia effects of the energy backup previously discussed. A possible solution to the pressure overshoot would be to establish an unrealistically and uneconomically conservative pressure setpoint.
A second method that is commonly incorporated to handle a complete load rejection is to initiate an automatic reactor trip whenever a serious load rejection occurs. This method has the disadvantage that the nuclear power system becomes available for the continued generation of electrical energy only after involved and expensive reactor startup procedures have been completed. Such a practice necessitates considerable expense, trouble and delay regardless of the fact that many of the problems which cause load rejections are temporary or are easily detected and rapidly remedied.
One cause for increased primary pressure is the sudden expansion of the primary coolant due to a rapid increase in its temperature. Thus, in order to prevent a reactor trip on high primary pressure, the magnitude of primary coolant temperature increase must be kept under a certain value. One way to accomplish this is to find a means of limiting the increase in primary coolant average temperatue to a value that is rathr independent of the power level. It is also desirable that this means be able to anticipate said increase when it is due to a disturbance in the load. Secondary pressure is a parameter which is very responsive to load changes. In addition, there is a magnitude correspondence between this type of secondary pressure increase and the resulting increase in primary coolant average temperature. Therefore, if the secondary pressure is controlled in the event of a load rejection so as to limit the magnitude of its increase, the primary pressure can be kept from reaching its trip value. The maximum allowable increase in secondary pressure for this purpose is almost constant through the power range. Hence, if the steam dump and/or load bypass valves are controlled so as to prevent the secondary pressure from increasing from its initial value by that maximum amount, a reactor trip on high primary pressure can be averted.